Particularly since the advent of aircraft gas turbine engines in the 1940's, a great amount of fruitful work has been done in the development of high temperature resistant alloys for use as components in such engines. To qualify for such use, an alloy must have a combination of a number of outstanding physical properties, one essential property being that of extremely high tensile strength at the elevated temperatures at which such gas turbine engines operate.
One group of alloys which has come into prominent use for such gas turbine engine components is that consisting of the nickel base alloys which additionally contain substantial amounts of chromium and cobalt and lesser amounts of other metals, particularly molybdenum, titanium, and aluminum. A typical such alloy would be one containing approximately 19% chromium, 14% cobalt, 4% molybdenum, 3% titanium, 1% aluminum, 1% iron, and the remainder substantially all nickel.
As is true for all alloys used as aircraft engine components, the specifications for such nickel are quite stringent. This, of course, adds to the cost of manufacturing the virgin alloy. Inherent to the manufacture of the engine components from the alloy stock, as by casting, machining and grinding operations, is the generation of scrap in the form of clean and contaminated metal turnings, grindings, casting scrap and the like. It has heretofore been the practice to remelt such scrap for reuse in making stock from which the turbine engine components are manufactured. However, where the scrap is contaminated or not readily sortable this is impractical and even where the scrap is clean and sortable it involves considerable expense since the stock which is made from or which includes the remelted scrap must, of course, meet the same stringent specifications as must be met by stock made entirely of virgin alloy. Included in the stringent specifications is the requirement for extremely high tensile strength, this characteristic being mandatory for aircraft engine components for obvious reasons.